Mostly need to work out some house rules to make the system less breakable--statted NPCs apparently suck. You can make a starting character on par at fighting than Ser Barristan/the Kingslayer/other amazing fighters. Then as they game goes on you can pump up their stats more, making them far better.

The House Creation system is a blast, at least.

edit: I've considered gurps, but we've used it extensively in the past and are interested in trying something new.

Is it the statted NPCs that suck, or they overpower new characters? Because I'd assume from the books that the game is more about house dynamics rather than fighting

Are your players all caught up with the books? Because you don't want to spoil them.

The setting is so tied to the unfinished narrative, I think it would be hard. Could try Alternate Game of Thrones. Make something else up...like a continent to the west that is all set on invading. Or Winter doesn't come and the Wall starts to melt.

Either way, what is great about the setting is there is a looming threat from afar...but the kingdoms are all sucktarded jerks so they aren't going to be ready. The campaign sets itself up for the first half: getting the kingdoms in order. Second half: face the threat with whatever forces you managed to rally

Storywise, 1 has read everything, 1 is finishing SoS, 2 have only watched the show.

I'm starting them off a year prior to the Tournament of Harrenhall during the False Spring. They'll be dealing with an insurrection on the lands of another bannerman of their liege house. Depending on how they handle it, they could gain standing with the Tyrells (they decided their house would be in the Reach,) possibly gain more land/territory...or things could go the other way and they could end up far worse off. This'll basically set up how much power/influence they'll have heading to the Tourney.

Depending on how that goes, I'll be altering the timeline based on their actions. They might alter events that start the Rebellion, or they might change the outcome of the war itself. It'll be fun reacting to them.